The present disclosure relates to a process and apparatus for producing aligned electrospun fibers, and more specifically to a process and attendant apparatus for forming electrospun fibers having nanoscale or micron-scale dimensions.
Electrospinning, which shares characteristics of both electrospraying and conventional solution dry-spinning of fibers, is a fiber-forming process that uses an electric charge to draw micron- or nano-scale fibers from a liquid feedstock. Advantageously, the process is non-invasive and does not require the use of coagulation chemistry or high temperatures to produce solid threads. These aspects make the process particularly well-suited to the production of fibers using large and complex molecules. Solventless electrospinning from molten precursors can also be performed, ensuring that no solvent is retained in the final product.
A conventional laboratory setup for electrospinning comprises a spinneret (typically a hypodermic syringe needle) connected to a high-voltage (5 to 50 kV) direct current (DC) power supply, and a grounded collector plate. A liquid feedstock (e.g., solution, suspension, melt, etc.) is passed through the needle tip using, for example, a syringe pump or a header tank that can provide a constant feed velocity, feed pressure, etc. The constant pressure feed, which is associated with the header tank, can be advantageous for lower viscosity feedstocks.
When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid droplet is charged, electrostatic repulsion counteracts the surface tension, and the droplet is stretched. At a critical point, a stream of liquid erupts from the droplet surface and forms a so-called Taylor cone.
If molecular cohesion of the liquid is sufficiently weak, the stream of liquid will break up into a plurality of droplets as the droplets are electrosprayed. On the other hand, if molecular cohesion of the liquid is sufficiently strong, stream breakup does not occur and a charged liquid jet is formed. The liquid jet may be attracted to other electrically charged objects at a suitable electrical potential.
As the jet dries out in flight, the mode of current flow changes from ohmic to convective as charge migrates to the surface of the fiber. The jet is elongated by a whipping process caused by electrostatic repulsion initiated at small bends in the fiber, and finally it is deposited on the grounded collector. The elongation and thinning of the fiber resulting from this bending instability can lead to the formation of fibers having a uniform diameter.
Applications for electrospun fibers include catalysis, filtration media, filler for fiber-containing composites, and scaffolds for tissue engineering. However, some fiber applications can be limited by the deposition pattern of the resulting fiber mat. Notably, alignment of the as-spun fibers would increase the number of applications for which the fibers were suited. Optical polarizers, for example, could be made using aligned fibers. Accordingly, it would be advantageous to provide a high-yield electrospinning process capable of providing aligned fiber collection and control over fiber dimensions while maintaining compatibility with a broad range of fiber-forming materials.
These and other aspects and advantages of the invention can be achieved via a fiber formation process that comprises forming a fiber stream comprising an electrically conductive fluid, the fiber stream being formed by a fiber-forming module that is biased at a first DC voltage, directing the fiber stream at a collector, the collector comprising a plurality of electrodes each electrically biased at a second DC voltage, attracting the fiber stream to successive ones of the electrodes by varying a time-dependent voltage applied to at least one of the electrodes, and depositing the fiber stream between successive pairs of the electrodes such that the deposited fibers are aligned between respective electrode pairs.
An apparatus for forming aligned electrospun fibers comprises a fiber-forming module, a fiber-collecting module having a plurality of electrodes, and a time-dependent voltage power supply, wherein the time-dependent voltage power supply is configured to provide a time-dependent voltage to one or more of the electrodes.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments that are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and together with the description serve to explain various principles and operations.